A heat-sensitive recording method has many advantages in that (1) no particular developing step is required, (2) if paper is used as a support, the recording material prepared can have a quality akin to that of plain paper, (3) handling of a recording material used is easy, (4) images recorded has high color density, (5) this method can be embodied using a simple and cheap apparatus, (6) no noise is generated upon recording, and so on. Therefore, heat-sensitive recording materials have recently come into wide use in the fields of a black and white facsimile and a printer. These heat-sensitive recording materials are prepared by coating a combination of a color producing agent and a color developing agent on a support, such as paper, synthetic paper, etc., and a record is made thereon through a process of heating with a thermal head on the basis of electric signals corresponding to an original.
In the above-described recording fields also, a requirement to get color hard copies with ease from data terminals of information equipments including a computer, a facsimile and so on has been increased with the rapid advance of information industry. As a method to fulfil such a requirement, an ink jet process and a heat-sensitive transfer process have been examined. In the ink jet process, however, a coloring agent and other ingredients in ink tend to cause stopping-up of a nozzle because of narrowness of the nozzle out of which coloring agent-containing ink is shot. Therefore, such a process has a serious defect that it lacks reliability of the records formed. In the heat-sensitive transfer process, on the other hand, ink on an ink sheet is dissolved imagewisely by heating, and transferred onto paper. Accordingly, in order to obtain, for example, a 4-color image four ink sheets, are required. That is, such a process is uneconomical since it requires a great number of ink sheets. In addition, a user must devote a ceaseless attention so that ink solutions, in case of the ink jet process, and ink sheets, in case of the heat-sensitive transfer process, may not become scarce in supply. That is to say, both the processes force bothersome supervision on users.
In contrast to the foregoing processes, a heat-sensitive recording process requires no bothersome supervision, and can provide highly reliable records. Therefore, in case a multicolor recording material is realized according to this process, the material can be free from the defects of conventional processes, and used with ease. In order to render a record multicolored, however, it is necessary to provide, over one support, color-producing mechanisms in number corresponding to that of colors to be produced, and to make the color-producing mechanism fulfil their respective functions under control. Although much efforts to meet the above-described necessities have so far been made, all materials proposed are still insufficient in respect of hue and color separation of produced colors.
For instance, there is a conventional method in which plural colored units are simply added one after another in proportion as applied heat energy is increased, and thereby the produced colors are mixed to cause changes in hue as accompanied by turbidity, as described in Japanese Patent Publication Nos. 19989/76 and 11231/77, Japanese patent application (OPI) Nos. 88135/79, 133991/80 and 133992/80 (The term "OPI" as used herein means an "unexamined published application"). As another conventional method, there is known one which introduces decolorization mechanisms, such that a color produced by a color-producing unit at a low temperature is decolorized by the reaction with a decolorizing agent at the same time as a color-producing unit capable of making a thermal response at a higher temperature produces its color, as described, e.g., in Japanese Patent Publication Nos. 17868/75, 5791/76, 14318/82 and 14319/82, and Japanese patent application (OPI) No. 161688/80.
In the above-described methods, not only the number of realizable hues is small, but also turbidity due to blur and color mixing is generated. Therefore, it is hard to say that the heat sensitive materials employed in the foregoing methods have satisfactory functions as a color hard copy. In particular, it is a fatal defect as a color hard copy that the number of hues of producible colors is small in principle. One material reason why the conquest of this defect has not been successful as yet consists in the following circumstances. That is, if an increase in number of producible hues can be achieved by simply increasing the number of fractions of heat energy applied for printing and, at the same time, making a large difference in energy between one fraction and another one, it suffices in principle for this purpose to design a heat-sensitive recording material provided with an increased number of heat-sensitive color-producing layers on the same support so as to correspond to various heat energies applied thereto respectively. In reality, however, there occur problems in respect of freshness keeping property (the so-called fog) of the recording material itself and so on, when heat energy applied for printing is extended to a lower region than usual, while when the energy is extended to a higher region, there arise new grave problems that images formed become sticky through their burning or fusion (e.g., a sticking phenomenon tends to occur), whereby the traveling facility of a thermal head upon printing is deteriorated, the span so thermal-head life become short, and so on. Accordingly, the largest allowable number of fractions of heat energy to be applied for printing has so far been two or so from the practical viewpoint of satisfactory color separation.
In the meantime, an opaque support, such as paper, synthetic paper or the like, has usually been employed as the support of a heat-sensitive recording material. This depends merely on the way of using the produced color images as reflected images to be read from one side.
As hitherto known examples of a heat-sensitive recording material provided with a substantially transparent support, mention may be made of those described in Japanese Patent Publication No.20151/'65, Japanese patent application Nos. 68875/85 and 184483/85. Therein, the transparent support was used for the purpose of obtaining a high quality image, especially excellent in contrast or luster, by observing the images recorded by heat from the side of the transparent support.
Still another example of providing heat-sensitive color-producing layers on both sides of a support is disclosed in Japanese patent application (OPI) No. 208298/82. This aims at reduction of loss in aspects of cost, keeping space of printed copies, and so on by printing records on both sides of an opaque support. Accordingly, all the materials cited above do not have any special contribution to obviation of the foregoing defects inherent in multicolored recording material.
Moreover, inventions, in which two or more coloring images are obtainable by using recording materials which comprises a transparent support having on each side a heat-sensitive recording layer differing in hue of the produced color, are disclosed in Japanese patent application Nos. 114431/'74, 3640/'75 and 4092/'85.
However, in these recording materials, coloring components and color developers are simply dispersed in the heat-sensitive layer as solid states, color-producing layers themselves become opague and no multicolor images clearly devided into each colors are obtainable, since light is scattered by above mentioned dispersed solids. In the disclosure of Japanese patent application No. 4092/'85, some descriptions, which means that each component was solved and coated in same layer to improve transparency of the thermocoloring layer, are found but so called fog occurs in this case since coloring of each component before printing occurs easily.
Therefore, all these known technologies are essentially insufficient for multicolor recording material since the number of colors which can be distinguished each other are small.
As the results of our energies research on the above-described defects, it has been found that improved heat-sensitive color image can be obtained by providing coloring layers which are essentially transparent and can produce colors differing in hue on each side of a transparent support, then the present invention has been completed.